This disclosure relates generally to a system and method for detecting failed heliostats in a concentrating solar field. In particular, the invention relates to an improved system and method for assessing heliostat operability by reflecting artificial light from heliostats onto camera imagers.
In Concentrating Solar Power (CSP) plants an array of heliostats reflect sunlight toward a receiver mounted atop a tower and containing a working fluid. The working fluid may be, for example, water or molten salts. One type of receiver transfers incident radiant energy to the working fluid to produce high-pressure, high-temperature steam, which may later be fed to a turbine for electrical power generation. Heliostats are generally mounted on the ground in an area facing or surrounding the tower. Each heliostat has a reflector: a rigid reflective surface such as a mirror that tracks the sun through the actuation of a heliostat drive mechanism about at least one axis. Sun-tracking involves orienting the reflector throughout the day so as to optimally redirect sunlight from the sun toward the receiver and maintain the desired temperature of the working fluid. The orientation of each heliostat may be changed by actuating at least one motor to a set position.
The power output of a CSP plant depends directly on the accuracy with which heliostats may reflect light onto a desired region of the receiver tower, as well as the reliability with which heliostats may be actuated to deliver flux; these metrics are known as the heliostat pointing accuracy and heliostat availability, respectively. In particular, heliostat availability may be defined as the fraction of heliostats which are able to deliver flux to their intended target on command at a given time. If a heliostat malfunctions or is otherwise out of service, it is unable to reliably reflect sunlight onto the receiver and so decrements the average availability of the field. The effect of a failed heliostat on the availability is determined according to the following formula:
      A    =          1      -              MTTR                  MTTR          +          MTBF                      ,where A is the average availability, MTTR is the mean time to repair a failed heliostat, and MTBF is the mean time between failures. The MTTR may be calculated as the sum of the mean time to detect that a heliostat is broken and the mean time to repair or replace it. Because faulty heliostats will continue to lie dormant or point flux at the wrong target until they are replaced or recalibrated, it is desirable to provide a means for detecting heliostat failures as quickly as possible.
Some causes of heliostat failures may be readily identified by electrical diagnostics. These may include complications such as motor malfunctions, short circuits, blown fuses, or improper wiring. The heliostats may be periodically queried for status updates, and heliostats which do not respond after a predetermined interval may be flagged for investigation. It is far harder to detect heliostats which remain capable of actuation and communication, but are no longer capable of delivering flux to the receiver accurately and on demand. This scenario may occur if a heliostat has a broken mirror, has experienced a mechanical failure such as a loose bearing or bolt, or has been disturbed by either personnel, field equipment, or environmental forces (such as wind).
Conventional techniques for monitoring heliostat operability typically include regularly scheduled tests in which each heliostat in the field is directed to reflect sunlight onto a target. If the heliostat is commanded to reflect light onto the target and is unable to do so in a satisfactory manner it may be flagged for further inspection and possible maintenance or repair. The shortcomings of this approach are that it may take a very long amount of time to test every heliostat in an entire field, and any heliostats that are undergoing routine pointing to a target rather than the receiver are unavailable for power production. An additional mechanism for determining heliostat field availability is to monitor the flux delivered from the heliostat field or a subsection thereof and look for a drop in power production from expected levels. This method may prove to be ineffective due to the dependency of flux delivery on a variety of factors including, but not limited to, cloud cover, natural variations in direct normal solar insolation (DNI), and reflector cleanliness. Passive monitoring of plant characteristics like flux delivery and receiver temperature do not assist in identifying which heliostats amongst possibly thousands are to blame, requiring additional time and costs for subsequent focused investigations. Accordingly, there is a need for a method of detecting failed heliostats that is reliable, swift, and does not negatively impact heliostat availability for power production.